Electronic circuits nowadays tend to require more than one direct-current (DC) voltage power supply and hence, many systems have been developed for providing power conversion, one of which is the charge pump. A charge pump, also known as a switched-capacitor converter, includes one or more pumping capacitors, an output stage having two or more diodes or switches, an output capacitor, and a control circuit for switching different current paths in the charge pump, and is operative to generate different output voltages stepped up or stepped down from a DC voltage. For instance, FIG. 1 is a circuit diagram of a charge pump configured as a voltage tripler. A voltage-tripler charge pump refers to a charge pump capable of generating an output voltage Vout up to three times as high as the DC input voltage Vin. The operation of a charge pump includes charging (energy storage) and discharging (energy transfer) of its pumping capacitor(s). For example, in FIG. 1, a charge pump chip 10 has a feedback pin FB connected to a feedback circuit 12 to receive a feedback signal FB, and a comparator 14 compares the feedback signal FB with a target value Vref to generate a comparison signal for a charge pump control logic 16 to determine an operation mode and control signals for drivers 18 and 20, and by using the drivers 18 and 20 to alternately switch different current paths and diodes D1, D2 and D3 to constrain the direction of current flow, charges and discharges pumping capacitors C1 and C2 to generate the output voltage Vout at an output capacitor Cout.
A charge pump usually has three or more operation modes, and the maximum pumping voltage of each mode is an integer times the source voltage. For instance, the charge pump shown in FIG. 1 has an original voltage (×1) mode 22, a voltage doubler (×2) mode 24, and a voltage tripler (×3) mode 26 for providing a maximum pumping voltage of 1×Vin, 2×Vin, and 3×Vin, respectively, as shown in FIG. 2. In the ×1 mode 22, the drivers 18 and 20 keep one terminal of each of the pumping capacitors C1 and C2 at the ground potential, so the pumping capacitors C1 and C2 function as the output capacitor Cout. In the ×2 mode 24, the driver 20 keeps one terminal of the pumping capacitor C2 at the ground potential such that the pumping capacitor C2 functions as the output capacitor Cout, and only the pumping capacitor C1 is used for pumping voltage. In the ×3 mode 26, the pumping capacitor C1 is charged to a certain voltage and then discharged, thereby pulling high the potential of the pumping capacitor C1 by Vin, i.e., the voltage of the voltage source Vin. As a result, a voltage higher than Vin is generated for charging the pumping capacitor C2. When the pumping capacitor C2 is discharged, recharging of the pumping capacitor C1 resumes, and at the same time, the potential of the pumping capacitor C2 is pulled high by Vin, thereby generating a voltage higher than 2×Vin for charging the output capacitor Cout. After that, the pumping capacitor C2 is recharged. The mode transition is accomplished by controlling the drivers 18 and 20 by the charge pump control logic 16 shown in FIG. 1.
In practice, however, the demand is not necessarily an integer times of the supply voltage Vin. In order to provide an adequate output voltage Vout, the charge pump must operate in a mode whose maximum pumping voltage is higher than the demand. For instance, if the demanded voltage is between two times and three times of the supply voltage Vin, the charge pump will operate in the ×3 mode. As shown FIG. 1, according to the feedback signal FB, the comparator 14 signals the charge pump control logic 16 to control the drivers 18 and 20 for the output voltage Vout to reach the target value. In general, a charge pump has the efficiencyη=(Vout×Iout)/(Vin×Iin),  [Eq-1]where Vout is the stable output voltage of the charge pump, Iout is the output loading current of the charge pump, Vin is the DC input voltage of the charge pump, and Iin is the input current of the charge pump. As the output voltage Vout is locked by the feedback loop, a charge pump will have an ideal maximum efficiencyη=Vout/(N×Vin),  [Eq-2]where N is the maximum multiplication factor of the charge pump. According to the equation Eq-2, the closer to the maximum pumping voltage N×Vin the output voltage Vout is, the more efficient the charge pump will be. For instance, the charge pump shown in FIG. 1 has a maximum efficiencyη=Vout/(3×Vin).  [Eq-3]If the charge pump shown in FIG. 1 operates to provide an output voltage Vout lower than its maximum pumping voltage 3×Vin, part of the power will be consumed by the elements D1, D2 and D3 of the output stage and converted into heat. The heat generated by the Schottky diodes D1, D2 and D3 will raise the internal temperature of the charge pump chip 10 if these diodes D1, D2 and D3 are integrated in the charge pump chip 10, as shown in FIG. 1. However, the same heat is less likely to affect the charge pump chip 10 if these diodes D1, D2 and D3 are outside the charge pump chip 10. In addition, the Schottky diodes, once integrated into the charge pump chip 10, tend to increase the area of the chip significantly and incur an excessively high parasitic resistance. Therefore, Schottky diodes are preferably used in the form of external elements when constituting the output stage of a charge pump; nevertheless, such use of Schottky diodes will increase the volume and costs of the resultant charge pump.
If the output stage is implemented by power MOSFETs instead, such as the switches SW1, SW2 and SW3 shown in FIG. 3, the output stage can be incorporated into a charge pump chip 30 without increasing the volume and costs of the resultant charge pump; apart from that, this charge pump will have better performance than the charge pump shown in FIG. 1. However, the charge pump chip 30 requires a more complicated charge pump control logic 32 for controlling the switches SW1, SW2 and SW3, and the power consumed by the switches SW1, SW2 and SW3 will still generate heat that increases the temperature inside the charge pump chip 30.